The unidentified sludge struck the patio of a home on Nassau Boulevard about 9:30 p.m. Thursday, a neighbor said Saturday.

Laurence Major Jr., who lives across the street, said he visited his neighbors minutes later and saw that the odorless substance had spotted their clothes, deck and patio furniture.

"He said he and his wife were outside enjoying a couple of beverages on the deck and all of a sudden . . . they got pelted," Major said. "As quick as it came, it left, and this happened right after a plane passed over."

FAA spokeswoman Kathleen Bergen said the investigation would begin Monday.

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Bergen couldn't immediately confirm whether the substance came from a passing aircraft.

Major's neighbors declined requests for comment. No other neighbors reported being struck.

The FAA said it was contacted by Malverne police Friday afternoon and will start the investigation by visiting the homeowner and taking a sample of the substance, Bergen said.

"This is very rare. More often we get reports of 'blue ice' falling in people's backyards" from airplane lavatory holding tanks, she said.

If they determine the substance came from an aircraft, investigators will examine records "for any flights over that area at that time, then track back to the aircraft operator and have them look at their aircraft to see if there were any leaking or maintenance issues," she said.